It is a common problem to want to know the position of a device whose position is being controlled by an actuator or some other means. For example, in the controls industry, devices such as valves having a valve stem or valve shaft which is movable by an actuator are used to control the flow of liquids and gasses associated with industrial processes of various types. In these applications it is common to want to know, at any given time, the precise position of the movable valve stem or valve shaft. This information allows improved understanding of the process and the control of the process.
A number of prior solutions have been proposed. Linear variable differential transformers (lvdt) can provide accurate position information. However they require a mechanical linkage and also generally use relatively high power. Potentiometers or other rotary transducers require a mechanical linkage and also have the disadvantage of a sliding electrical contact which can cause long term reliability issues. Optical coding schemes make use of a coded element with opaque and transparent sections to provide digital data inputs to an array of sensors positioned to measure the light passing through the sections. While optical coding devices do not require a mechanical linkage, the optical approach only works well in very clean environments and is therefore not applied in many industrial environments.
These conventional contact-based position sensing methods require a direct mechanical linkage or translation of the moving component to a transducer, from which the physical position is then derived. Such systems suffer from at least two primary shortcomings. One is that mechanical linkages are prone to wear, causing the reported position to be unpredictable and inaccurate. Another is that mechanical translation of the components movement path to a transducer introduces position reporting errors due to uncorrectable non-linearities in the transformation. Previous approaches have addressed this problem for the specialized case of linear motion; however, rotary (circular) motion and motion that defines an arbitrary path that is neither linear nor circular is also needed in the position sensing field.
Thus a need exists for a reliable position determining apparatus that does not require a mechanical linkage between the apparatus and the movable member, is relatively insensitive to environmental factors, and robust enough to be applicable to non-linear motion.